Dielectric, infrared, and Raman response of undoped SrTiO3 ceramics: Evidence of polar grain boundaries - art. no. 184111

Thorough Raman and infrared (IR) reflectivity investigations of nominally p ure SrTiO3 ceramics in the 10-300 K range have revealed a clear presence of the polar phase whose manifestation steeply increases on cooling. The Rama n strengths of the Raman-forbidden IR modes are proportional to omega (-alp ha)(TO1)(alpha approximate to 1.6) where omega (TO1) is the polar soft mode frequency. No pronounced permittivity dispersion is observed below the sof t mode frequency so that, as in single crystals. the static permittivity is essentially determined by the soft mode contribution. A theory is suggeste d which assumes a frozen dipole moment connected with the grain boundaries which induces the polar phase in the grain bulk in correlation with the bul k soft-mode frequency. This stiffens slightly the effective soft mode respo nse and reduces the low-temperature permittivity compared to that of single crystals. Moreover. the polar soft mode strongly couples to the E-g compon ent of the structural soft doublet showing that the polar axis is perpendic ular to the tetragonal axis below the structural transition which is shifte d to 132 K in our ceramics. Whereas the TiO6 octahedra tilt (primary order parameter) below the structural transition corresponds to that in single cr ystals, much smaller A(1g)-E-g splitting of the structural soft doublet sho ws that the tetragonal deformation (secondary order parameter) is nearly 10 times smaller, apparently due to the grain volume clamping in ceramics.